As the universe of HIV-host complexes has now dramatically expanded, it is essential to validate and understand their functions in the context of virus replication, where all viral proteins are present. Knockdown technologies have been the mainstay for testing functional relevance and will be applied extensively to the Vif, Vpu, Vpr, PR, Tat, and Rev projects. One example in which experiments in the virus context have uncovered an unexpected connection between viral proteins involves the gene regulatory circuits controlled by Tat and Rev. Maintaining the proper activities of these proteins during the viral life cycle is critical; for example, slight changes in Tat expression can lead to switch-like behavior and can drive the virus into a latent state [141]. Because analyzing the individual activities of Tat and Rev is complicated by their overlapped reading frames, viruses were generated with unconstrained Tat and Rev genes placed in the nef region. The engineered Tat viruses replicate normally [69], but surprisingly, the Rev viruses do not (Fig. 11). The defect results from Rev overexpression or its incorrect timing, as Rev in these viruses exports the unspliced mRNAs too efficiently, before the spliced mRNAs encoding Tat are expressed and thus the viruses are Tat deficient. This type of Tat/Rev feedback has been reported [142], but how HIV evolves Tat and Rev sequences to maintain the correct balance of activities in the context of overlapped reading frames is an open question.